This invention relates to a variable resistor unit for high voltage (hereinafter referred to as "high-voltage variable resistor unit"), and more particularly to a high-voltage variable resistor unit called a focus pack which is adapted to be used for adjusting a focus voltage of a CRT (cathode ray tube) or the like or a screen voltage thereof.
In general, such a high-voltage variable resistor unit is mounted on a transformer casing of a flyback transformer. A high-voltage variable resistor unit of which an applied voltage is decreased is conventionally constructed in such a manner that a variable resistance circuit pattern for a variable resistance and a fixed resistance circuit pattern for a fixed resistance called a bleeder resistance which is connected in series to the variable resistance circuit pattern are formed on a single insulating substrate. Unfortunately, the high-voltage variable resistor unit of this type, when the applied voltage is increased, tends to cause discharge between the resistance circuit patterns and/or between each of the resistance circuit patterns and an electrode. In view of the problem, an approach has been conventionally employed wherein the fixed resistance called a bleeder resistance which is positioned on a high-voltage side is arranged on another insulating substrate to provide a fixed resistance circuit board, which is then housed in a second board receiving chamber arranged in an insulating casing separately from a first board receiving chamber for receiving a variable resistance circuit board.
For example, a high-voltage variable resistor unit disclosed in Japanese Utility Model Application Laid-Open Publication No. 116005/1991 is so constructed that a fixed resistance circuit board is received in a second board receiving chamber provided in juxtaposition to a first board receiving chamber for receiving a variable resistance circuit board therein. Generation of any crack at an insulating casing causes discharge to readily occur between a circuit pattern on the fixed resistance circuit board and an external conductive section. In order to solve the problem, the high-voltage variable resistor unit disclosed in the publication is so constructed that the fixed resistance circuit board is supported by means of a holder so as to increase a creeping distance between a high potential section on the fixed resistance circuit board and a wall of the second board receiving chamber. Also, Japanese Utility Model Application Laid-Open Publication No. 32504/1992 discloses a high-voltage variable resistor unit including a holder of a box-like shape which is provided at one end thereof with an opening and adapted to receive a circuit board therein, resulting in being increased in dielectric strength characteristics with respect to an exterior.
In the conventional high-voltage variable resistor unit described above, the variable resistance circuit board received in the first board receiving chamber is charged on a rear surface thereof with epoxy resin, followed by curing of the resin to provide an insulating resin layer, and the second board receiving chamber is charged with resin for molding the fly-back transformer, followed by curing of the resin.
Arrangement of the second board receiving chamber for receiving the fixed resistance circuit board therein in juxtaposition to the first board receiving chamber for receiving the variable resistance circuit board therein leads to a disadvantage of significantly increasing an outer dimension of the insulating casing. In order to reduce the outer dimension, it is proposed to vertically partition the insulating casing into an upper compartment and a lower compartment, so that the variable resistance circuit board is received in the upper compartment and the fixed resistance circuit board is arranged in the lower compartment. Unfortunately, this renders insulation between the variable resistance circuit board and the fixed resistance circuit board substantially difficult, so that an increase in serviceable voltage causes discharge to readily occur therebetween.
Further, the above-described operation of charging epoxy resin of soft properties on the rear surface of the variable resistance circuit board to form the insulating resin layer leads to an increase in the number of steps required for assembling the high-voltage variable resistor unit, as well as an increase in the amount of insulating resin required for insulation.